During President Ali Abdullah Saleh's four months in exile after an assassination attempt in June the country he has ruled for 33 years has remained in a political deadlock.

Despite the nine-month standoff between the Yemeni regime and anti-government activists, Saleh has managed to cling on to power while he is out of the country, in Saudi Arabia, receiving treatment for burns he suffered during the explosion. Authorities have suggested at times that Saleh would consider either a transition of power or a power-sharing agreement with some opposition figures who are demanding an end to his three decades of autocratic rule.

But fierce and deadly clashes have broken out over the past two days due to frustration at the lack of progress towards democracy, alongside escalating government fears about a resurgent protest movement tapping into a regional momentum generated by the fall of Muammar Gaddafi in Libya.

On Monday night Yemen's capital, Sana'a, was a desperate scene of gunfire and chaos as the standoff between anti-regime activists and government forces spilled over into violence that observers say could spread to other parts of the country.

Witnesses said government troops stationed throughout the central city and snipers perched on rooftops were responsible for much of the carnage, which has caused more than 50 deaths since Sunday, with hundreds more injured.

After earlier clashes on Monday, security forces launched an intensive sweep through central areas of the capital, which had been a focal point for protesters.

As fighting intensified demonstrators appear to have overrun a compound of presidential guards inside the capital, then moved on to other parts of the city.

By nightfall Sana'a international airport had been closed and heavy shooting could be heard in many areas of the capital.

"Yemen is on a knife edge," said Amnesty International's Middle East and north Africa deputy director, Philip Luther. "Those who have been protesting peacefully for change are increasingly frustrated.

"Meanwhile, eruptions of violence point to a growing risk of civil war. The Yemeni authorities must stop the use of excessive force before the violence spirals out of control."

Protesters have grown increasingly angry at Saleh's failure to listen to their demands. The opposition was angered last week by Saleh's move to deputise his vice-president to seal a Gulf-brokered and US-backed deal which would allow the president to step down but remain immune from prosecution. Saleh has three times indicated he would sign the deal, then reneged.

Some demonstrators called for a Libya-style international intervention to help oust Saleh.

The uprising has been inspired by the same sentiments that have unseated three north African dictators since January and have brought instability to Syria and Bahrain. Yemenis have bemoaned the fact that their uprising has only briefly captured the regional imagination and has not received anything like the same international attention as Egypt and Libya.

Opposition and government groups inside the country, and regional observers, now fear events over the past few days have reached a turning point in this heavily armed and deeply unstable nation.

The US and Saudi Arabia both fear what would come next if Saleh left power. Both states point to an al-Qaida insurgency inside Yemen that could gain greater access to political power in the event of a vacuum.

Yemeni officials again claimed al-Qaida elements, along with members of the powerful Ahram tribe, were behind the latest violence. Officials have consistently denied firing on demonstrators and said anyone found to be doing so would be held accountable. "The government of Yemen expresses its sorrow and condemnation for all acts of violence and bloodshed as those happened yesterday in Sana'a," foreign minister Abu Bakr al-Kurbi told the UN human rights council.

The US embassy called on all sides to "exercise restraint" and not escalate the violence further. However, its low-key response was rounded on by rights groups and activists inside Yemen.

"Nine whole months protesting in the streets under the burning sunlight and still no one is appreciating our peaceful efforts," said Nujood Saleh, a youth activist in Sana'a. "The Libyan revolution succeeded by the use of force while we are still suffering. We insist on peaceful strategies to achieve freedom and democracy."

"The international community cannot continue to put its security concerns and fears about al-Qaida before human rights considerations," said Amnesty International's deputy regional director, Philip Luther. "It must be made clear to the Yemeni authorities that protesters should not be targeted for exercising their rights. The abuses being committed by Yemeni forces are totally unacceptable and must cease."